The present invention relates to a method of calibrating a sensor, in which at most as many so-called influence variables affecting the measuring result are considered, as measurable quantities are detected by the sensor, the set of the influence variables being composed of at least one disturbance variable influencing the measurement and of at least one target value which is to be determined from the quantities being measured. The present invention relates in particular to a method of calibrating an eddy-current sensor for distance measuring and for material testing.
For measuring nonelectrical, primarily mechanical quantities, modern automated manufacturing processes make increasing use of electronic sensors as control sensors, in particular noncontacting distance sensors. Besides the almost nonreactive detection of the measurable quantities, further advantages of such sensors lie in a response practically free of delay, in the insensitivity to vibrations, dust, moisture, and chemically aggressive vapors, as well as in the extensive freedom from maintenance.
Important representatives of this group are the eddy-current sensors, which are especially suitable for measuring short displacements and distances under difficult environmental conditions. Similarly to the instance of a plurality of other measuring methods, in the instance of distance measuring by means of the eddy-current measuring principle, the quantities being measured (i.e., measurable quantities) include the resistive (R) and reactive (X) components of the impedance (Z) of the object of measurement (i.e., the target). The resistive and reactive components are influenced not only by the target quantity, the distance, but are to the same extent dependent on a whole series of other influence variables such as the conductivity (.sigma.) and/or permeability (.mu.) of the target. Besides the factors which are known in the measurement or are adjustable, such as, for example, the geometry of the coil and object of measurement, frequency and amplitude of the coil current, the influences of mostly unknown material properties of the object being measured create the greatest problems. Examples are primarily inhomogeneities in the conductivity and permeability, as well as temperature fluctuations and material defects. The conductivity and permeability of the object of measurement or the target are hereafter described as disturbance variables.
It is the object of the present invention to provide a method of calibrating any desired sensor, such as an eddy-current sensor, so as to suppress disturbance variables.